Continuous casting apparatus with means automatically controlling the holding vessel discharge



July 29, 1969 J. H. wlLsoN 3,457,985

CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMATICALLY CONTROLLING THEHOLDING VESSEL DSCHARGE Filed Dec. 16, 1966 3 Sheets-Sheet 1 35 .5f/Paya/144.4 A

1 GEA/wmf? July 29, 1969 1. H. wlLsoN 3,457,985

CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMATICALLY CONTROLLING THEHOLDING VESSEL DISCHARGE Filed Dec. 16, 1966 5 Sheets-Sheet 2 mvzimok.JAMES ML150/y H. WILSON CONTINUOUS CASTING APPARATUS WITH MEANS AUTOMAT3,457,985 ICALLY July 29, 1969 1 CONTROLLING TEE HOLDING vEssELDISCHARGE 16. 1966 3 Sheets-Sheet 5 Filed Dec.

United States Patent O 3,457,985 CONTINUOUS CASTING APPARATUS WITH MEANSAUTOMATICALLY CONTROLLING THE HOLDING VESSEL DISCHARGE James H. Wilson,Franklin Township, Westmoreland County, Pa., assignor to United StatesSteel Corporation, a corporation of Delaware Filed Dec. 16, 1966, Ser.No. 602,319 Int. Cl. B22e 19/04, 25/00, 11/10 U.S. Cl. 164-155 5 ClaimsABSTRACT OF THE DISCLOSURE Continuous casting apparatus including aholding vessel above a flow-through mold, in which automatic control ofthe discharge valve of the vessel is responsive to the height of themetal level in the vessel. When the metal level falls below the normaloperating value, the discharge valve is automatically closed. A rise ofthe level effects opening of the valve. An electrical system providing awide dead band produces the desired spacing between valveopen andvalve-closed conditions.

This invention relates to continuous casting apparatus, and moreparticularly to continuous casting apparatus comprising a ladle formolten metal, a water-cooled continuous casting mold, and a holdingvessel having a bottom discharge valve between the ladle :and the mold,in which provision is made for automatically controlling the opening andclosing of the discharge valve in the holding vessel.

Apparatus for the continuous casting of metals and particularly steelnearly always includes -a ladle for molten metal, and an open-endedtubular water-cooled continuous casting mold in which a metal castingmay be formed. Because of the necessity for maintaining a substantiallyuniform molten metal level in the mold and the difficulties inaccurately controlling the pouring rate when teeming molten metaldirectly from a ladle into a mold, it is frequently advantageous toinclude a holding vessel between the ladle and the mold. This holdingvessel provides a reservoir for molten metal, and as a consequence, asubstantially uniform flow of metal from the holding vessel into themold can be maintained even when there are marked fluctuations in theteeming rate from the ladle into the holding vessel.

The holding vessel may be either a tundish or a degassing vessel. Theuse of a tundish between the ladle and the mold in continuous steelcasting is well known in the rart. The purpose of -a tundish is tosmooth out of the flow rate of molten metal into the mold. A continuousin-line degassing vessel having a molten metal inlet at its upper endand a valve controlled discharge opening at its lower end performs thesame function as a tundish in smoothing out the metal flow rate, `andmore importantly, also removes considerable quantities of undesirablegases, par ticularly oxygen, from the molten metal before it is teemedinto the mold.

When starting up a continuous casting apparatus of the type described,molten metal is first poured from the ladle into the holding vessel. Itis desirable to establish :a predetermined operating level of moltenmetal in the holding vessel before metal is teemed from this vessel intothe mold. When the holding vessel is a degassing vessel operated underhigh vacuum, it is necessary to provide a sufficient depth of moltenmetal to provide a barometric seal 3,457,985 Patented July 29, 1969 lCeand thereby prevent the passage of air from the atmosphere into theevacuated vessel. In order to establish the desired liquid level, it isnecessary to keep the discharge valve and the holding vessel closeduntil this level has been reached. Thereafter this discharge valveshould remain open throughout the casting operation, unless the level ofmolten metal in the holding vessel falls to some predetermined level,considerably lower than that at which the discharge valve is opened,below which further operation of the system is undesirable. In the caseof a vacuum degassing vessel, the discharge valve should be shut in theevent that the liquid level falls so low that there is danger ofbreaking the barometric seal. In the case of tundish, it is desirable toclose the discharge valve when the tundish has been substantiallyemptied of molten metal, leaving mostly non-metallic impurities floatingon the surface of the metal. By closing the discharge valve of thetundish before these non-metallic impurities are teemed into the mold,the accumulation of such impurities in the mold, where they wouldadversely effect the quality of the casting, is prevented.

In some instances it is desirable to provide a holding vessel having apair of inlet openings :and a single discharge opening, so that castingoperations may continue indefinitely, or in any case for longer than thelength of time required to empty ya single ladle, by alternately teemingmolten metal from a pair of ladles. It is desirable in such apparatus toprovide means for switching from one ladle to the other, so that moltenmetal is teemed only one ladle at a time, and for automaticallycontrolling the movements of the stopper rod of the teeming ladle inresponse to the level of molten metal in either the holding vessel orthe mold.

It is an object of this invention to provide an apparatus forautomatically opening the discharge valve of a holding Vessel locatedbetween the ladle and the mold of a continuous casting apparatus when apredetermined height of molten metal is reached.

It is a further object of this invention to automatically close theholding vessel discharge valve in the event that the molten metal levelfalls to some predetermined level which is substantially below that atwhich the discharge valve was opened, while keeping this valve open whenthe metal level falls only a slight degree below the level at which thevalve was opened.

A further object of this invention is to provide an automatic liquidlevel responsive ladle stopper rod control for a continuous-continuonssystem having two ladles, which control system includes means forswitching from one ladle to the other.

According to this invention, there is provided a continuous metalcasting `apparatus including at least one ladle, a holding vesseltherebelow having a bottom discharge opening controlled by a valve, :anda continuous casting mold below the holding vessel, in which the openingof the discharge Valve of the holding vessel is automatically controlledby -a signal which is ultimately derived from load cells which measurethe amount of molten metal in the holding vessel and hence the metallevel therein. The discharge valve is conveniently a hydraulicallyoperated valve in which the hydraulic control system for operating thevalve includes ya piston :and a solenoid-operated four-way valve whichcontrols movements of the piston. An amplified signal from the loadcells generates an error signal when the actual level of molten metal inthe holding vessel differs from a pre-set level, the polarity of theerror signal being determined by the direction of deviation and theamplitude of the error signal being determined by the magnitude ofdeviation. A bi-stable magnetic amplifier amplifies the error signal andthereby controls a relay which alternately energizes one or the other ofthe two solenoids controlling the four-way valve without ever energizingboth solenoids simultaneously.

The present invention also provides means for automatically closing thedischarge valve of the holding vessel when the molten metal leveltherein drops to a second predetermined level, considerably below thefirst level at which the discharge valve was opened, and below whichfurther operation of the holding vessel is undesirable. Since there isconsiderable difference between the valve opening level and the valveclosing level, the electrical system of this invention includes meansproviding a wide deadband, so that the discharge valve will not openuntil the predetermined opening level is reached, but will then remainopened unless the predetermined closing level is reached, in which casethe discharge valve is closed to prevent further metal teeming.

This invention also provides a degassing vessel or tundish having a pairof inlet openings at its upper end and a single discharge opening at itslower end, each of the inlet openings being adapted to receive moltenmetal, so that molten metal may be teemed into the holding vessel fromone ladle at a time and, when the first vessel is substantially empty,it may be shut off and removed and metal immediately teemed from theother ladle without interruption. This type of operation is known ascontinuous-Continous operation. More especially this invention providesmeans for switching from one ladle to the other and for automaticallycontrolling the teeming of molten metal from the teeming ladle inresponse to the level of molten metal in either the holding vessel orthe mold.

Referring now to the drawings:

FIG. 1 is a diagrammatic view of a preferred apparatus according to thisinvention, showing the elements of the continuous casting apparatusschematically and the elements of the electrical control and hydrauliccontrol systems in block diagrams.

FIG. 2 is a schematic diagram of the electrical apparatus of thisinvention which controls the opening and closing of the holding vesseldischarge valve.

FIG. 3 is a schematic electrical diagram of the apparatus forcontrolling the teeming of molten metal from a pair of ladles into theholding vessel.

Referring now to FIG. 1, 11 and 12 are a pair of bottom pour ladleshaving stopper rods 13 and 14 respectively for controlling the teemingof molten metal therefrom. Molten metal is teemed from one ladle at atime into a vacuum degassing vessel 15 having a pair of inlet openings16 and 17, a connection 18 to a source of high vacuum, and a bottomdischarge opening 19 which is controlled by sliding gate valve 20, whichhas an orifice 21. The sliding gate valve may be that shown anddescribed in the copending application of James T. Shapland, Ser. No.453,730, filed May 6, 1965, now Patent No. 3,352, 465, issued Nov.14,1967.

Below the discharge opening 19 of holding vessel 15 is an open-endedtubular water-cooled flow-through continuous casting mold 22 which isadapted to receive molten metal from holding vessel 15 at its upper end,and to discharge a partially solidified metal casting from its lower end(not shown).

The stopper rods 13 and 14 of ladles 11 and 12 respectively arecontrolled by identical hydraulic control systems 23 and 24respectively, shown in -box diagrams in FIG. 1. These control systemsmay be similar to the hydraulic control system shown and descirbed inCarleton Patent No. 2,832,110, issued Apr. 29, 1958, and more especiallyin FIG. of that patent.

The sliding gate valve 20 may be controlled by .a hydraulic system whichis essentially similar to that shown in FIG. 5 of the aforesaid CarletonPatent No. 2,832,110.

Referring to FIG. 1 herein, where the components of this system areshown diagrammatically, the sliding gate valve 20 is controlled byhydraulic cylinder 25 which has a reciprocating piston 26. The positionof this piston is controlled by the aforesaid hydraulic system, whichincludes a hydraulic reservoir 27 for hydraulic fiuid, a pump 28, and afour-way valve 29 which is controlled by a dual solenoid operator 30comprising a pair of solenoids, only one of which can be energized at atime.

This invention provides a novel control system for controlling the dualsolenoid 30 in response to the level of molten metal in degassing vessel15, as indicated by load cells 31. An on-ofl switch 31a, which is closedduring normal operation of the apparatus herein described, opens andcloses the circuit between load cells 31 and the system which controlsdual solenoid operator 30. A load cell signal representing the averagereading of the load cells 31 is amplified by amplifier 32 which drives amotor 33, which produces an error signal Ee in error signal generator34. This error signal generator 34 may be like that shown in FIG. 4 ofMilnes Patent No. 3,204,460, issued Sept. 7, 1965. The error signal Eehas a polarity which is determined by the direction of deviation betweenthe actual liquid level and a desired preset liquid level in vessel 15,and a magnitude which is determined by the amount of deviation. When theactual liquid level is above the preset level, Ee is positive, and thiserror signal places bistable magnetic amplifier 35 in the on state,energizing relay 36 which controls the current flow in electrical system37. This electrical system 37 controls the dual solenoid operator 30which in turn controls the hydraulic system which actuates the movementsof valve 2t).

Referring now to FIG. 2, the error signal generator 34 is shown ingreater detail with reference to a schematic wiring diagram. The purposeof error signal generator 34 is to form an error signal voltage Ee whosepolarity is determined by the direction of deviation between the actualliquid level and a preset liquid level in vessel 15, and whose amplitudeis determined by the magnitude of this deviation. Error signal generator34 includes a DC power source such as a battery, a pair of slide wireresistors 41 and 42 which are in parallel and connected to the oppositepoles of battery 40, and a bridge circuit which includes a pair ofconductors 43 and 44 having contacts 45 and 46 respectively, which areslidable along resistors 4.1 and 42 respectively. The position ofcontact 45 and hence the potential of conductor 43 is governed by thelevel of molten metal in degassing vessel 15. The load cells 31 indicatethe liquid level in degassing vessel 15 and this signal is amplified byamplifier 32 which supplies the power input to motor 33, which has ashaft 47 for moving contact 45 to a position along slide wire resistor41 which corresponds to the actual liquid level of molten metal invessel 15. A reference potential is established in conductor 44 bysetting contact 46 at a desired position on slide wire 42 whichcorresponds to the liquid level desired in vessel 15. When conductors 43and 44 are at the same potential, the bridge is balanced and no currentflows through either conductor. However, when conductors 43 and 44 areat different potentials, an error signal voltage Ee is generated, andconnection of conductors 43 and 44 will cause a current to fiowtherethrough. As indicated above, the polarity of error signal Eeindicates the direction of deviation between the actual liquid level andthe desired preset liquid level, and the arnplitude of Ee isproportional to the extent of such deviation, as indicated by thedifference in voltage between conductor 43 and conductor 44. A switch 48in series with conductor 44 is provided for starting up the apparatus.

The electrical control system of this invention includes a bistablemagnetic amplifier 35 having a single output which is either on or ofiVarious bistable magnetic amplifiers are known in the art. The bistablemagnetic amplifier 35 may be Type 100C008 made by the NorbatrolElectronics Corporation, Pittsburgh, Pa., and described in their CatalogCX60-3, dated 'October 1961. The bistable magnetic amplifier 35 includesa magnetic core 50, an AC power source 51 which is typically suppliedwith 110 volts AC, and a plurality of windings each having a pair ofterminals for attachment of electrical components.

Conductor 43 is connected in series with the DC control Winding 52 ofmagnetic amplifier 35. The current through winding 52 is proportional tothe error signal voltage Ee. Both are taken as positive when the actualliquid level in vessel is above the preset liquid level.

The current in DC control winding 52 is normally opposed by a biascurrent in Ibias winding 53. The bias circuit 54 which controls thiscurrent also includes a battery 55 and a variable resistor 56. Thepolarity is such that the bias current is always negative. That is, thebias current opposes the .primary DC control current in winding 52 whenthe actual liquid level is above the preset liquid level, i.e., when Eeand the control current in winding 52 are positive. Thus the magnitudeof the error signal voltage E., must be large enough for the controlcurrent to overcome the bias current before the magnetic amplifier 35 isplaced in the on state.

The magnetic amplifier 35 also includes a bandwidth control comprising avariable resistor 57 connected to a winding 58 for controlling the widthof the deadband which in turn determines the difference between theliquid level at which valve opens and the level at which valve 20closes. The width of the deadband increases as the resistance ofresistor 57 increases. Generally the resistance of resistor 57 is chosenso that valve 20 closes at a liquid level considerably lower than thelevel at which it opens.

The output of magnetic amplifier 35 is a single output which includes arectifier. A relay 36 is in the external circuit of the amplifieroutput. When the algebraic sum of the DC control current in winding 52and the bias current in winding 53 is zero or negative, the magneticamplifier 35 is in the ofi state and relay 36 is de-energized. But whenthe algebraic sum of these currents is positive, amplifier 35 is in theon state and relay 36 is energized.

To start up a continuous casting operation, switch 4S is closed, andmolten metal is teemed from ladle 11 into degassing vessel 15 with thedischarge valve 20 closed. At the start, the error signal voltage E., isnegative. The current flowing through winding 52 is also negative. Inthis state, the negative current in winding 52 is summed with the biascurrent in winding 53, which in all cases shall be considered to benegative. The bistable magnetic amplifier 35 is in the off state as longas the resultant of the control current in winding 52 and the current inwinding 53 is negative. Relay 36 remains closed while the amplifier 35is in the o state.

As the molten metal level continues to rise, it reaches the point atwhich the potentials in the liquid level con trolled conductor 43 andthe reference conductor 44 are equal so that the error signal Ee iszero. The resultant current is still negative and therefore the magneticamplier 35 remains in the off state. As the liquid level further rises,the current in winding 52 becomes positive. When this current issufficient to offset the negative current in bias winding 53, the core50 is saturated with a polarity such that the magnetic amplifier 35 isthrown into the on state. This causes relay 36 to be energized.Energization of relay 36 opens Contact 36A and closes contact 36B inelectrical system 37. Electrical system 37, 4which has an independentpower supply (typically 110 volts AC), controls dual solenoid operator30 (FIG. 1), which includes a pair of opposed solenoids 61 and `62controlling valve 29. Energization of relay 36 deenergizes solenoid I61and simultaneously energizes solenoid 62. This moves the four-way valve29 to the position causing the movement of piston 26 in the directionwhich opens 6 discharge valve 20. Molten metal is then teemed into mold22.

`Once valve 20 has been opened, it is caused to remain open as long asnormal operation of the continuous casting apparatus continues, eventhough the actual liquid level Iin vessel 15 may from time to time fallbelow the level at which the valve 20 was opened. This is accomplishedby means of variable resistor 57 which serves as a deadband control asalready mentioned. As long as the molten metal level in degassing vessel15 is sufficiently above the ybarometric height of molten metal so thatthere is no danger of breaking the vacuum in vessel 15, it is desirableto keep discharge valve 20 open. The height at which valve 20 must beclosed is considerably lower than the height at which this valve isopened, and therefore a wide band Width is used.

Referring now to FIG. 3, the control system for operating the stopperrods 13 and 14 of ladles 11 and 12 respectively will now be described.This control system is generally similar to the control system shown anddescribed in the co-pending application of Tiskus et al., Ser. No.353,696, now Patent No. 3,300,820, issued lan. 31, 1967, except that thesystem of said co-pending application has been modified herein so that apair of ladle stopper rods may be controlled. Also, in a preferredernbodiment, the control of the stopper rods is made in response to thelevel of molten metal in degassing vessel 15, rather than in response tothe level of molten metal in mold 22.

FIG. 3 illustrates in `electrical schematic form a liquid level controlcircuit 70 which responds to the error signal Ee to produce a controlsignal Ec which controls the positions of ladle stopper rods 13 and 14.

The stopper rod control 24 for stopper rod 13 of ladle 11 includes apair of solenoids 71 and 72 which control a four-way valve, whosemovements control the admission of hydraulic fluid to a hydrauliccylinder `which raises and lowers stopper rod 13 of ladle 11. Thestopper rod control 23 for stopper rod 14 of ladle 12 works in exactlythe same way. This stopper rod control 23 includes a pair of solenoids73 and 74 which control stopper rod 14. Energization of solenoids 71 and73 causes upward movement of stopper rods 13 and 14 respectively, Whileenergization of solenoids 72 and 74 causes downward movement of stopperrods 13 and 14 respectively.

Metal is teemed from only one of the ladles 11 and 12 at a time. Thismay be accomplished by means of a multiple pole relay 80, whoseoperation is controlled by a switch 81 located at any convenient point,such as an operators console. IRelay includes three normally closedcontacts 80A, 80B, and 80C, and three normally open contacts 80D, 80Eand 80F. Contacts 80A and |80B control the flow of current throughcontacts 99B and 102B respectively to solenoids 71 and 72 respectively.Contacts 99B and 102B are controlled by relays 99 and 102 respectively,which will be described subsequently. Contact 80C controls the flow ofcurrent from power supply line 100 to solenoid 74. Contacts 80D and 80Econtrol the fiow of current through contacts 99B and 102B respectivelyto solenoids 73 and 74 respectively, while contact SGF controls currentfrom power line 100 to solenoid 72. When the contacts in multiple poleswitch 80 are in their normal positions as shown, solenoid 74 isenergized, holding stopper rod 14 of ladle 12 in the closed position,While stopper rod 13 of ladle 11 is free to move up and down in responseto the alternate energizations of solenoids 71 and 72. By closing switch81 the relay 80 (FIG. 3) is energized and the normally open closedcontacts 80A, 80B and 80C are opened. This contacts 80D, 80E and 80F areclosed, and the normally causes solenoid 72 to be energized, holdingstopper rod 13 of ladle 11 in the closed position, While up and downmovements of stopper rod 14 of ladle 12 are controlled by solenoids 73and 74.

The error `signal Ee is fed to a liquid level control circuit 70, whichresponds to the error signal Ee, in order to provide a control signal Ecthat adjusts the position of stopper rod 13. In this fashion a servoloop is obtained by which stopper rod 13 is adjusted as a function ofthe liquid level in degassing vessel 15.

The control signal Ec has an intermittent component Ecl that energizesthe solenoid 71 to cause an increase in liquid fiow rate and anintermittent component EQ2 that energizes the solenoid 72 to cause adecrease in the flow rate. The control circuit 70 is designed so thatthe components Ecl and EQ2 cannot be on simultaneously, although bothmay be off at the same time.

The error signal Ee cannot simply be amplified in order to control thesolenoid operated valve, since that simple a system would produceserious overshooting and hunting in the control of the liquid level inthe vessel 15. The -control circuit 70 must anticipate the changes inliquid level in the vessel by also being responsive to to the rate ofchange of liquid level and modify the control signal Ec accordingly.Furthermore, the loop response time is such that it is desirable for thecontrol signal Ecl plus EQ2 to be intermittent. The intermittent controlsystem causes the changes in the position of stopper rod 13 to be madein small steps, thus permitting sufficient time to elapse between thesteps so that the change in pour rate will be refiected in the errorsignal The liquid level control circuit 70 employs a dual outputmagnetic amplifier 90 to control an excitation of solenoids 71 and 72,or alternatively solenoids 73 and 74. The dual output magnetic amplifier90 has an AC power source 91 and a pair of DC input control windings 92and 93, so that whatever signal is developed across winding 92 isalgebraically summed with whatever signal is developed across winding 93to determine whether output A or output B or neither output is turnedon. The output circuit of magnetic amplifier 90 includes a commonnegative conductor 94 and a pair of positive output conductors 95 and96. The magnetic amplifier 90 can be turned on or off by operation ofswitch 97 in the AC supply 91. In the off state both relays 99 and 102remain deenergized regardless of the error voltage amplitude orpolarity.

When the magnetic amplifier is in state A, a 4closed loop is completedbetween common conductor 94 and conductor 95. When the amplifier is instate B a circuit is completed between conductor 94 and conductor 96.The logic of magnetic amplifier 90 is such that both outputs A and Bcannot be on at the same time but both outputs can be off at the sametime. The operation of this circuit can be more easily understood bystarting at the back end first and comprehending how the output of themagnetic amplifier affects the solenoids 71, 72, 73 and 74.

When the magnetic amplifier is in state A, relay 99 is energized,thereby closing the normally open relay contacts 99A and 99B. Theclosing of contact 99B completes a circuit from the line 100, which maybe an AC power source, through the now closed contact 99B, the solenoid71, and through the normally closed contact `101A back to line 100. Thusthe throwing of the magnetic amplifier 90 into state A causes anincrease in the rate of molten metal flow from ladle 11 because of theenergization of solenoid 71. At the same time the closing of contact 99Aenergizes the time delay relay 101. This time delay relay 101 is set tooperate at a predetermined length of time, for example, approximatelyone-half second after it has been energized, so that one-half secondafter the contact has been closed, the time delay relay contact 74Aopens, thus breaking this circuit to solenoid 71. In this manner, thestopper rod 13 is lifted only by the amount that would be determined byone-half second of flow of liuid from the hydraulic system. Thus only asmall step change is made in the pour rate of molten metal from theladle 11.

If the magnetic amplifier is in state B, then the relay '102 isenergized, thereby closing contacts 102A and 102B. The closing ofcontact 102B energizes the solenoid 72, thereby lowering the stopper rod13 and decreasing the flow of molten metal from ladle 11. Thesimultaneous closing of contact 102A energizes the time delay relay 103and thereby causes the contact 103A to open in one-half second whichthen de-energizes solenoid 72. With the solenoid 72 de-energized, theentire system continues to function with a metal fiow rate into thedegassing vessel 15 that has been decreased to an extent determined bythe amount which the stopper rod descends in one-half second ofoperation of the hydraulic system.

When an error signal Ee exists, it charges the capacitor 104 at a ratewhich is controlled by the setting of the variable resistor 105. Oncethe variable resistor '105 has been set, the rate of charge of thecapacitor 104 is then strictly a function of the magnitude of the errorsignal Ee. The polarity of charge of the capacitor 104 is a function ofthe polarity of the error signal Ee. The capacitor 104 is connected inseries with the variable resistor 106, across the control winding 92 ofthe dual output magnetic amplifier 90. When the voltage across thecapacitor 104 reaches a pre-selected value (the value being governed bythe setting of the variable resistor 106) it produces a signal acrossthe control winding which causes the magnetic amplifier 90 to provide anelectrical output at either A or B, depending on the polarity of thecharge across the capacitor 104. Thus either the relay 99 or the relay102 is operated by the magnetic amplifier 90 depending entirely upon thepolarity of Ee.

As has been described previously, the excitation of either relay 99 orrelay 102 will excite the coil of time delay relay 101. The time delayrelay 101 can be adjusted manually to operate over a range of timeperiods; the time period selected in the embodiment illustrated isapproximately one-half second. When one-half second has elapsed not onlydoes the contact 101A open to immediately de-energize whichever of thetwo solenoids 71 and 72 was energized, but in addition the contact 101Bcloses to discharge the capacitor 104. By thus discharging the capacitor104, the signal across the control winding 92 is eliminated, the dualoutput magnetic amplifier 90 ceases to have an output, and the cycle maybe started all over. As long as the error signal Ee persists, thecapacitor 104 will conttinue to be charged to turn on the appropriaterelay 99 or 102 to cause the appropriate half second excitation of thesolenoid controlled hydraulic valve in stopper rod control 23. Thisvalve in turn will cause the pour rate of molten metal from ladle 11 tovary in a direction tending to decrease the magnitude of error signalEe. Thus the frequency of pulsing through control winding 92 isproportional to the error signal Ee, and, for a given signal Ee, `thefrequency of pulsing can be adjusted by the setting of the variableresistor 105 which then sets the charging rate of the capacitor 104.

In order to avoid overshooting the mark when employing the controlcircuitry just described in connection with the response of the magneticamplifier 90 to the er ror signal Ee, it is desirable to furtherregulate the pouring rate in accordance with the rate of change of theliquid level in the degassing vessel 15. If the liquid level, forexample, is below the pre-selected level but is increasing at a rapidrate, it is desirable to decrease rather than increase the pour rate toprevent overshooting and to obtain accurate liquid level control andrapid correction of the error. The circuitry connected with the controlwinding 93 is employed to achieve this purpose of overriding the controldictated by the error signal Ee when the rate of change of the liquidlevel is sufficiently rapid and is in a direction tending to decreasethe error signal Ee.

A potentiometer arrangement 110 is employed for controlling the currentthrough DC control winding 93. This potentiometer arrangement includes abattery 111, a slide wire resistor 112, and a variable resistor 113 inseries with battery 111 and slide wire resistor 112. A slider contact114 is moved along a slide wire resistor 112 in such a fashion that theposition of the contact 114 along the resistor 112 corresponds to thelocation of the liquid level in the degassing vessel 15. This may beachieved by having the sliding contact 114 controlled in the samefashion as is the slider arm 74 in FIG. 4 of the aforesaid Milnes PatentNo. 3,204,460. If the contact 114 is stationary, a voltage is developedacross capacitor 115 which is equal and opposite to the voltage oncontact 114 so that no current flows through variable resistor 116 orwinding 93, which are in parallel. However, as long as the contact 114is moving, a current will iiow through the circuit that includes thevariable resistor 116 in order to charge the capacitor 115. The currentiiowing through the resistor 116 develops a signal across the controlwinding 93, which signal is summed with the signal across the controlwinding 92. The various parameters in this portion of the circuit, suchas the magnitude of the battery 111, the capacitance of the capacitor115, resistances of the variable resistors 112 and 113, as well as thenumber of turns on the control winding 93, can be adjusted or selectedso that the signal across the winding 93 will dominate the signal acrossthe winding 92 and will be the controlling signal as long as there is anappreciable rate of change of the contact 114. Thus, for example, if theliquid level is below that desired and the level is rising rapidly, thesignal developed across the control winding 93 will oppose that acrossthe control winding 92 so as to cause the dual output magnetic amplifier90 to be switched to state B instead of state A, thereby reducing ratherthan increasing the pour rate. If, however, the rate of change of liquidlevel is zero, or very small, then the signal across the winding 93 willbe minor compared with the signal across the winding 92, and the stateof the dual output magnetic amplifier 90 will be determined solely bythe polarity of the error signal Ee.

The time delay relay 103 performs the same function for the circuitassociated with the control winding 93 as does the time delay relay 101for the control element 92. When either the relays 99 or 102 areenergized, the time delay relay 103 becomes energized by the closing ofeither contact 99A or 102A. The time delay relay 103 is preset to haveits contact 103A close a predetermined length of time after it has beenenergized, which in this case may be in the order of one-half second.Thus onehalf second after a change in the position ofnstopper rod 13 hasbeen initiated, the relay contact 103A will close and short out thevariable resistor 116, thereby dropping the signal on the control coil93 to zero and turning off whichever state the dual output amplifier 90may have been in.

It should be remembered that when one of the states (that is, state A orstate B) is turned olf in this dual output amplifier 90, that the otherstate is not thereby automatically turned on, and thus the turning offof one of the states simply means that there is no output at all andthus no energization for either the relay 99 or the relay 102, which inturn means that the stopper rod 13 will remain in the position to whichit was last moved.

The switch 97 is normally closed but may be opened when it is desired todisenable the stopper rod control circuit.

The description with reference to FIG. 3 has been with particularreference to the control of teeming in ladle 11. When ladle 11 issubstantially empty, it is necessary to switch over from ladle 11 toladle 12 in order to continue the teeming of molten metal into degassingvessel 15 without interruption. By manual closing of the switch 98, thenormally closed contacts of relay 80 are opened,

and the normally open contacts are closed. When this is done, solenoid73 will be actuated in place of solenoid 71 to increase the stopper rodopening of stopper rod 14 in ladle 12, and energization of solenoid 74will cause partial closing of stopper rod 14 in ladle 12.

Instead of generating the error signal Ee from a signal from load cells31, the error signal can be obtained from a signal which is obtainedfrom a bank of thermocouples (FIG. l) and fed to a liquid leveldeviation detector 121, as more fully described in Milnes Patent No.3,204,- 460. A switch 122 is placed in the liquid level detectioncircuit and is in the off position when the signal from load cells 31 isused to generate the error signal.

While this invention has beeny described with respect to specificembodiments thereof, it will be understood that variations can be madeby those skilled in the art without departing from the scope and spiritof this invention.

What is claimed is:

1. In a continuous metal casting apparatus including a bottom pour ladlehaving a stopper rod controlling the discharge of molten metaltherefrom; a holding vessel below said ladle having an upper inletopening for receiving molten metal from said ladle, a bottom dischargeopening, a discharge valve controlling said discharge opening, and valveoperating means :for automatically controlling the opening and closingof said discharge valve; and an open-ended How-through continuouscasting mold beneath the discharge opening of said holding vessel, theimprovement comprising:

(1) means for continuously measuring the level of molten metal in saidholding vessel;

(2) means for generating an error signal whose polarity indicates thedirection of deviation lof the measured level of molten metal in saidholding vessel yfrom a preset level and whose magnitude indicates theextent of such deviation;

(3) single output amplifier means for providing an output signal Kwhenthe polarity of said error signal indicates lan actual liquid levelabove the preset level in said holding vessel and when said error signalexceeds a predetermined magnitude, and

(4) means for controlling said valve operating means 'in response tosaid output signal.

2. Apparatus according to claim 1 in which said amplitier means is abistable magnetic amplier.

3. Apparatus according to claim 1 in which said amplifier means has adeadband whereby said discharge valve is opened when the level of moltensteel exceeds a rst predetermined level and is closed when the level ofmolten steel falls below a second predetermined level which is lowerthan said first level.

4. Apparatus according to claim 1 in which said valve operating meanscomprises 4a hydraulic system having a source of fluid under pressure, afour-way valve, a doubleacting piston controlled by said four-way valve,and a pair oflsolenoids for controlling the position of said Ifour-wayva ve.

5. Apparatus according to claim 1 in which said means for controllingsaid Valve operating means includes a circuit having a power source, apair of opposed solenoids, one of which causes said discharge valve toopen and the other of which causes said discharge valve' to close, arelay in the output circuit of said amplifier means and adapted to beenergized by said output signal, and a pair of contacts operated by saidrelay for controlling the energization of said solenoids, one of saidcontacts being normally open and the other normally closed so that saidsolenoids cannot be simultaneously energized.

References Cited UNITED STATES PATENTS Re. 25,892 ll/l965 Tarukawa222-56 X 2,381,505 8/ 1945 Lindholm 222-56 (Other references onfollowing page) 11 i 12 3,133,726 5/1964 Tarukawa 222.-56 X J. SPENCERoVERHoLsER, Primary Examiner 2,905,989 9/1959 Black 164-155 3,300,8201/1967 Tiskus e1 a1. 164-155 R- D. BALDWIN, Asslstant Exammer FOREIGNPATENTS 5 n U.s. c1. XP. 732,115 6/1955 Great Britain. 164-64, 281;222-56 965,167 7/1964 Great Britain.

